求解，关于Thread的join问题

[Vault Boy]

public class DownLoadFrame extends Thread{
Socket socket4 = null;
int port4 = 21000;
String serverIP4;
FileOutputStream fos;
DataInputStream dis;
OutputStream os4;
InputStream is4;

private JFrame downloadframe = new JFrame();
private JList<String> downloadlist = new JList<String>();
private JScrollPane downloadpane = new JScrollPane(downloadlist);
String[] filelist;
String selectedfile;

public void run() {
try {
filelist=null;
selectedfile=null;

InetAddress inet = InetAddress.getLocalHost();
serverIP4=inet.getHostAddress();
socket4 = new Socket(serverIP4,port4);
System.out.println("文件下载服务连接成功！");

os4=socket4.getOutputStream();
is4=socket4.getInputStream();

int filelistlength = is4.read();
filelist = new String[filelistlength];

for(int i=0;i<filelistlength;i++) {
byte[] bt = new byte[1024];
int n = is4.read(bt);
filelist[i]=new String(bt,0,n);
}

Runnable downloader = new Runnable() {
public void run() {
downloadframe.setSize(300,200);
downloadframe.add(downloadpane);
downloadframe.setVisible(true);
downloadframe.setTitle("双击选择下载文件");
downloadlist.setListData(filelist);


MouseListener mouseListener = new MouseAdapter() {
public void mouseClicked(MouseEvent e) {
if (e.getClickCount() == 2) {
selectedfile = downloadlist.getSelectedValue();
downloadframe.dispose();
}
}
};
downloadlist.addMouseListener(mouseListener);

}
} ;
Thread download = new Thread(downloader);
download.start();
download.join();//等待子线程结束

System.out.println(selectedfile);

求解，按道理来说调用了join()方法之后要等到这个子线程结束了之后才会继续主线程，我的设想是在窗口程序内选择了一个值，把这个值输入给变量，然后关闭窗口，再把这个值打印出来，可是这里程序运行时join像是根本没有起作用一样，直接输出了一个空字符null，这是为什么呢？